1. Field of the Invention
The invention relates in general to a method of using a phase shifting mask, and more particularly to a method for forming different patterns using one phase shifting mask (PSM).
2. Description of the Related Art
Currently, since the resolution of the exposure process can be improved by using a phase shifting mask (PSM), photolithography is performed with a PSM in the integrated circuit. A patterned PSM comprises a transparent region and an opaque region, and the transparent region further comprises a phase shifting region and a non-phase shifting region. When an optical beam passes through the phase shifting region, the phase of the wave of the optical beam is reversed by the phase shifting region. Therefore, a wave of an optical beam passing through a photomask in the phase shifting region is different from that passing through the non-phase shifting region of the PSM with a optical phase difference. When photolithography is performed, because of interference between the wave of the optical beam passing through the phase shifting region and that passing through the non-phase shifting region, the images projected close to one another can be completely separated. Hence, the resolution of the exposure process is greatly improved. A Levenson PSM is a conventional PSM and is also called an "alternating PSM".
Conventionally, the phase shifting region and the non-phase shifting region of a Levenson-type phase shifting mask can be adjusted by the thickness of the photomask substrate. It is necessary to perform the E-beam writing process and the etching process twice to manufacture a Levenson-type phase shifting mask. A first E-beam writing process and a first etching process are used to define a transparent region and an opaque region on a photomask substrate. A second E-beam writing process and a second etching process are used to form a trench in the photomask substrate to define a phase shifting region. The phase of the wave after the optical beam passes through the photomask relates to the thickness of the photomask substrate that the optical beam has passed through. Therefore, the depth of the trench should be accurately controlled to obtain 180 degree optical phase difference between the wave of an optical beam passing through the phase shifting region and that passing through non-phase shifting region.
A structure of the Levenson-type phase shifting mask comprises a phase shifting layer formed on a chromium film over a quartz substrate. The phase shifting layer includes MoSi.sub.2 O.sub.x N.sub.y or SiO.sub.x N.sub.y. When a light passes through the Levenson-type PSM, the phase shifting layer shifts the light wave phase by 180 degrees (negative phase). Light passing through an exposed portion of the quartz substrate forms the light wave phase by 0 degrees (positive phase). Since phase shifting regions and non-phase shifting regions are cross-set, a light intensity of zero occurs at the interface between the positive phase and the negative phase. A light intensity with a better pattern resolution is obtained.
However, device dimension is shrunk so that photomask fabrication becomes more difficult and more complex than before. Thus, more photolithography processes are used while fabricating semiconductor devices, making the masks more expensive.